You're looking for a free download of Metro 2035 by Dmitry Glukhovsky in TXT format.
Here's a brief summary of the story:
As Artyom navigates the dangers of the metro, he discovers that a new threat is emerging: a charismatic leader who seeks to unite the various factions of the metro under his rule. Artyom must journey through the treacherous world of the metro, confronting various enemies and making difficult choices to prevent a new war from breaking out.